Method and device for calibration of dual-axis tilt meter

ABSTRACT

The invention is directed toward a subsurface gravity measurement device and a method for calibrating the same that includes a tilt meter and a gravity sensor. The method includes associating tilt information produced by the gravity sensor as a function of a relationship between tilt information produced by the tilt meter and a correction parameter. The tilt meter produces tilt data, and the gravity meter produces gravity data, corresponding to the tilt data The tilt data and gravity data is fitted to a polynomial equation that has a plurality of initial coefficients associated therewith. The initial coefficients include information concerning the correction parameter. The correction parameter is derived as a function of the initial coefficients.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention concerns downhole/borehole gravity metersthat sense variation in gravitational fields. More particularly, thepresent invention is directed to a dual-axis tilt meter employed insubsurface oil exploration and retrieval.

[0003] 2. Background Art

[0004] Gravity meters have been employed to measure characteristics ofgeologic formation and are used in the exploitation of hydrocarbonreservoirs found in geologic formations, commonly referred to as oilexploration and retrieval. Specifically, exploitation of hydrocarbonreservoirs involves characterizing oil, gas, and/or water.

[0005] Characterization of oil and gas in a hydrocarbon reservoir can bemonitored as a function of gravity by analyzing borehole and surfacegravity data. To that end, borehole gravity data is used to map out thevertical distribution of oil and gas at a well and surface gravity datacan be employed to characterize the area of distribution.

[0006] Typically, borehole gravity surveys involve measuring gravity atdiffering locations in a borehole, which typically correspond todifferent vertical distances from the surface. The difference in gravity(Δ{overscore (g)}) and the difference in vertical distance (Δ{overscore(z)}) between two successive locations yield sufficient information todetermine the bulk density of an area of the geologic formation adjacentto the borehole. The information concerning bulk rock density is mappedto determine the vertical distribution of oil and gas as the reservoiris exploited.

[0007] As a result, gravity measurements are typically monitored in themicrogal (10⁻⁶ cm/s²) or nano-g range to ensure useable data thatprovides an indication of untapped pockets of oil or gas in theaforementioned area. This level of resolution in gravity measurementsrequires a highly precise gravity sensor and carefully implementedmeasuring techniques. For instance, the gravity sensor must be orientedso that the sensitive axis of the sensor is parallel to a vertical linerepresenting the direction of gravity and commonly referred to as theplumb line.

[0008] To assist in properly aligning gravity sensors, many gravitymeters include a tilt meter. The tilt meter is employed to minimizeinclination of the gravity sensor's sensitive axis with respect to theplumb line. The tilt meter, however, must be properly aligned withrespect to the gravity sensor for best results. Further, the relativealignment of the tilt meter and the gravity sensor should be checkedperiodically as the same may vary due to shock or vibration that occursduring field operations, particularly during transportation andhandling. Another related issue is the sensitivity of the tilt meter,which may also change over time due to aging of the tilt meter andelectronic components. In order to correct for these problems, the tiltmeter is preferably calibrated prior to a gravity survey.

[0009] An exemplary gravity meter that includes the tilt meter/gravitysensor combination is available from SCINTREX® under the trade nameCG-3. The CG-3 is a surface gravity meter, and the calibration methodemployed includes calibrating each tilt axis separately, with the axisorthogonal thereto remaining fixed. The gain and offset of thecalibration on each tilt axis are computed separately. The deviationfrom the plumb line is computed first, providing a calibrated offsetvalue. The gain or sensitivity is then computed as a result of thecalibrated offset value.

[0010] A drawback with the aforementioned calibration technique is thatthe same is difficult to employ in a subsurface gravity measurementtool, because calibrating the gravity tool based upon a fixed orthogonalaxis introduces errors.

[0011] A need exists, therefore, to provide a method and a system tocalibrate a gravity tool to provide accurate subsurface gravitymeasurements.

SUMMARY OF THE INVENTION

[0012] The invention provides a method for calibrating a subsurfacegravity measurement device having a tilt meter and a gravity sensor. Themethod comprises associating tilt information produced by the gravitysensor as a function of tilt information produced by the tilt meter andan initial correction parameter; producing tilt data with the tiltmeter, and gravity data, corresponding to the tilt data, with thegravity data being produced by the gravity sensor; fitting the tilt dataand the gravity data to a polynomial equation, with the polynomialequation having a plurality of initial coefficients associatedtherewith, the initial coefficients including information concerning theinitial correction parameter; and deriving a correction parameter as afunction of the initial coefficients.

[0013] The invention provides a method for calibrating, with respect toa plumb line, a gravity measurement device having a tilt meter and agravity sensor. The method comprises associating tilt informationproduced by the gravity sensor as a function of a relationship betweentilt information produced by the tilt meter and an initial correctionparameter; orientating the tilt meter in a plurality of differingangular positions with respect to the plumb line, defining tilt data;measuring, with the gravity sensor, gravity information at each of theangular positions, defining gravity data; fitting the tilt data and thegravity data to a polynomial equation, with the polynomial equationhaving a plurality of initial coefficients associated therewith, thecoefficients including information concerning the correction parameter;determining values for the plurality of initial coefficients using aleast-squares regression; and deriving a correction parameter as afunction of the coefficient values.

[0014] The invention provides a subsurface gravity measurement device,comprising a body; a tilt meter connected to the body to produce tiltdata concerning angular positions the tilt meter forms with respect to adirection of gravity, with the direction of gravity defining a plumbline; a gravity sensor connected to measure the gravity and to produceinformation corresponding thereto, defining gravity data, with theinformation being a function of an angle the gravity measurement deviceforms with respect to the plumb line, defining tilt information; aprocessor in data communication with both the gravity sensor and thetilt meter; and a memory in data communication with the processor, thememory including a computer readable program to be operated on by theprocessor that includes a first subroutine to define a relationshipbetween the tilt information produced by the gravity sensor and both thetilt data and an initial correction parameter, and a second subroutineto fit the tilt data and the gravity data to a polynomial equation, withthe polynomial equation having a plurality of initial coefficientsassociated therewith, the initial coefficients including informationconcerning the correction parameter, and a third subroutine to derive acorrection parameter as a function of the initial coefficients.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a simplified plan view of the gravity measurement devicein accordance with the present invention;

[0016]FIG. 2 is a graph showing the orientation of the X-axis of agravity sensor included in the gravity measurement device of FIG. 1 withrespect to a direction of gravity;

[0017]FIG. 3 is a graph showing the orientation of the Y-axis of agravity sensor included in the gravity measurement device of FIG. 1 withrespect to a direction of gravity;

[0018]FIG. 4 is a flow diagram showing a method of calibrating thegravity meter shown in FIG. 1 in accordance with one embodiment of thepresent invention; and

[0019]FIG. 5 is a flow diagram showing a method of calibrating thegravity measurement device shown in FIG. 1 in accordance with analternate embodiment of the present invention.

DETAILED DESCRIPTION

[0020] Referring to FIG. 1, a gravity measurement device 10 inaccordance with one embodiment of the present invention is suitable fordownhole gravity measurements typically employed in the exploitation ofhydrocarbon reservoirs found in naturally occurring geologic formations.To that end, gravity measurement device 10 includes a gravity sensor 12and a tilt meter 14 connected to a common body 16 to fix the relativeposition of gravity sensor 12 and tilt meter 14. Gravity sensor 12 maybe any known gravity sensor in the art, such as a spring-mass-type,falling body/free-fall-type, pendulum-type and the like. Tilt meter 14may be any tilt meter known in the art, such as an electronicpendulum-type, electronic bubble-type and the like. Tilt meter 14 iscapable of sensing angles of inclination in two orthogonal axes. To thatend, tilt meter 14 includes an X-axis tilt sensor 14 a and a Y-axis tiltsensor 14 b. Gravity sensor 12 and tilt meter 14 are coupled together sothat tilt meter 14 is able to sense any change in the angle ofinclination of gravity sensor 12. As a result, both gravity sensor 12and tilt meter 14 are fixedly attached to body 16. Operation of gravitymeasurement device 10 is controlled by a processor 17 operating on acomputer readable program stored in a memory 19 that is in datacommunication with processor 17. The processor 17 is in datacommunication with both gravity sensor 12 and tilt meter 14.

[0021] Referring to both FIGS. 1 and 2, to ensure gravity sensor 12provides accurate measurements, the angle of inclination that gravitysensor 12 has with respect to a direction of gravity, {overscore (g)},referred to as a plumb line, is determined. Specifically, gravity sensor12 is sensitive to the position of the two transverse axes, both ofwhich are orthogonal to the plumb line. Assuming that the plumb lineextends along the Z-axis, the transverse axes are defined to be alongthe X and Y-axes. To obtain accurate measurements of gravity, it isimportant to determine the inclination angle θ_(x), between the gravitysensor 12's X-axis and the plumb line. Similarly, accurate gravitymeasurements by gravity sensor 12 are also dependent upon knowing theinclination angle, θ_(y), between the gravity sensor 12's Y-axis and theplumb line, shown more clearly in FIG. 3.

[0022] Referring to FIGS. 1-3, to that end, tilt meter 14 is employed todetermine the angles of inclination θ_(x) and θ_(y). However, thisassumes that the sensing axes of tilt meter 14 are aligned with thesensing axes of gravity sensor 12. This is not always the case. Assumingthat perfect alignment always exists between the sensing axes of thetilt meter 14 and the sensing axes of gravity sensor 12 is problematic,as this alignment may change over the operational life of gravitymeasurement device 10. This introduces errors in the gravity measurementmade by gravity sensor 12.

[0023] To abrogate errors in gravity measurements made by gravity sensor12, a calibration procedure is employed to define the relationshipbetween inclination angles θ_(x) and θ_(y) and tilt measurements X_(m)and Y_(m) sensed by tilt meter 14.

[0024] For example, the relationship between θ_(x) and θ_(y) and themeasured tilt angles X_(m) and Y_(m) may be defined as follows:

θ_(x) =k _(x)(x _(m)+ε_(x))  (1)

θ_(y) =k _(y)(y _(m)+ε_(y))  (2)

[0025] where k_(x) and ε_(x) are the gain and offset values,respectively, associated with measurements along the X-axis. Thevariables k_(y) and ε_(y) are the gain and offset values, respectively,associated with measurements along the Y-axis. The values k_(x), ε_(x)are correction parameters that define the difference between theinclination θ_(x) and tilt measurement X_(m). Likewise the valuesk_(y),ε_(y) are correction parameters that define the difference betweenthe inclination θ_(y) and tilt measurement Y_(m). These correctionparameters are referred to collectively as correction parameters k,ε.

[0026] Were gravity sensor 12 aligned so that θ_(x) and θ_(y) were 0°,the gravity measurement, R₀, would be a maximum value. From theforegoing it can be shown that the measured gravity, R_(m), is definedas follows:

R _(m) =R _(o) −{overscore (g)}(1−cos θ_(x) cos θ_(y)),  (3)

[0027] where {overscore (g)} is the average gravity in the region. Sinceangles θ_(x) and θ_(y) are small and measured in radians, a cosineapproximation is employed that abrogates the higher order terms so thatEquation (3) may be expressed as follows: $\begin{matrix}{R_{m} = {R_{o} - {{\overset{\rightarrow}{g}\left\lbrack {\left\lbrack {\frac{\theta_{x}^{2}}{2} + \frac{\theta_{y}^{2}}{2}} \right\rbrack - \frac{\theta_{x}^{2}\theta_{y}^{2}}{4}} \right\rbrack}.}}} & (4)\end{matrix}$

[0028] Again the smallness of angles Θ₁ and Θ₂ allows abrogation of thehigher order terms so that equation (4) may be expressed as follows:$\begin{matrix}{R_{m} = {R_{o} - {{\overset{\rightarrow}{g}\left\lbrack \left\lbrack {\frac{\theta_{x}^{2}}{2} + \frac{\theta_{y}^{2}}{2}} \right\rbrack \right\rbrack}.}}} & (5)\end{matrix}$

[0029] Substituting the values for θ_(x) and θ_(y) from Equations (1)and (2), Equation (5) is expressed as follows: $\begin{matrix}\begin{matrix}{R_{m} = {{{- \frac{\overset{\rightarrow}{g}\quad k_{x}^{2}}{2}}x_{m}^{2}} - {\overset{\rightarrow}{g}\quad ɛ_{x}k_{x}^{2}x_{m}} - {\frac{\overset{\rightarrow}{g}\quad k_{y}^{2}}{2}y_{m}^{2}} - {\overset{\rightarrow}{g}\quad ɛ_{y}k_{y}^{2}y_{m}} -}} \\{{{\frac{\overset{\rightarrow}{g}}{2}\left( {{k_{x}^{2}ɛ_{x}^{2}} + {k_{y}^{2}ɛ_{y}^{2}}} \right)} + {R_{o}.}}}\end{matrix} & (6)\end{matrix}$

[0030] It is seen that Equation (6) is a polynomial having the generalform as follows:

R _(m) =ax _(m) ² +bx _(m) +cy _(m) ² +dy _(m) +e.  (7)

[0031] The polynomial has a plurality of coefficients associatedtherewith: a, b, c, d and e. The values of coefficients a, b, c, d and emay be ascertained employing a linear least-squares regression. Knowingthe values of the coefficients a, b, c, d and e, the correctionparameters k,ε may be derived by solving for k and ε in both the X-axisand the Y-axis as follows: $\begin{matrix}{{k_{x}^{2} = {- \frac{2a}{\overset{\rightarrow}{g}}}},} & (8) \\{{ɛ_{x} = {- \frac{b}{\overset{\rightarrow}{g}k_{x}^{2}}}},} & (9) \\{{k_{y}^{2} = {- \frac{2c}{\overset{\rightarrow}{g}}}},} & (10) \\{ɛ_{y} = {- {\frac{d}{\overset{\rightarrow}{g}k_{y}^{2}}.}}} & (11)\end{matrix}$

[0032] The value for the correction parameter may be substituted intoEquations (1) and (2) to determine the inclination angles θ_(x) andθ_(y). Thereafter, the inclination angles are included in the gravitymeasurement performed by gravity sensor 12, using well-known techniques.In this fashion gravity measurement device 10 is calibrated to provideaccurate gravity measurements in any environment. Further accuracy couldbe ensured by appropriately weighting the measurements R_(m). Forexample, gravity measurements, R_(m), made at the varying angles X_(m)and Y_(m) may be weighted so that the weight given to a particularlygravity measurement, R_(m), is inversely proportional to the standarddeviation of the gravity associated with the measurement.

[0033] Referring to FIGS. 1 and 4, as discussed above, the operation ofgravity measurement system 10 is under control of processor 17 operatingon a computer readable program stored in memory 19. To that end, thecomputer readable program may be programmed using any language known inthe computer art to include the subroutines necessary to carryout thecalibration of the gravity measurement device 10 in accordance with thepresent invention.

[0034] In one embodiment, the computer readable program stored in memory19 would facilitate a method of calibrating gravity measurement device10 by associating tilt information produced by gravity sensor 12 as afunction of tilt information produced by the tilt meter 14 andcorrection parameters kε, at step 100. At step 102, tilt meter 14produces tilt data and gravity sensor 12 produces gravity data thatcorresponds to the tilt data Specifically, a plurality of gravitymeasurements, R_(m), are made by gravity sensor 12 at pairs of tiltangles X_(m) and Y_(m). The number of differing tilt angles X_(m) andY_(m) at which gravity measurements are made is typically no less thanfive, i.e., m=1-5. At step 104, the tilt data and gravity data producedat step 102 are fitted to a polynomial equation that has a plurality ofinitial coefficients, a, b, c, d and e, associated therewith. Thisfitting is performed employing a least means-squared regression. Initialcoefficients, a, b, c, d and e, include information concerningcorrection parameters kε. At step 106, correction parameters k,ε arederived as a function of initial coefficients a, b, c, d and e. At step108, inclination angles, θ_(x) and θ_(y), are determined based upon thecorrection parameters, kε. Thereafter, gravity measurement device 10 maybe employed to make gravity measurements based upon the known value ofinclination angles θ_(x) and θ_(y).

[0035] In accordance with another embodiment of the present invention,an additional step may be included in the method discussed with respectto FIG. 4 in which a measurement of the goodness fit of the polynomialin Equation (7) is determined. The goodness fit measurement may beperformed by employing a chi-square statistic defined as follows:

χ²=Σ|(R _(mi)−(ax _(mi) ² +bx _(mi) +cy _(mi) ² +dy _(mi) +e))|²/σ_(i)²,  (12)

[0036] wherein σ_(i) ² is the weighting factor mentioned above. Thesmaller the value of χ² the better the goodness fit of the underlyingpolynomial equation and, hence, the accuracy of the inclination anglesθ_(x) and θ_(y).

[0037] Considering that the value of χ² decreases as the number ofgravity measurements increase, in another embodiment, a normalizedchi-square statistic, χ_(v) ² may be employed. In this manner, thenormalized chi-square statistic is defined as follows:

χ_(v) ²=χ²/(N−n),  (13)

[0038] where N is the number of gravity measurements, and n is thenumber of unknown parameters. In this case the number of unknownparameters is five: a, b, c, d and e. The closer the value of χ_(v) ² isto 1, the more accurate the determination of inclination angles θ_(x)and θ_(y).

[0039] Referring to FIG. 5, in yet another embodiment of the presentinvention, the χ² measurement may be analyzed over time during theoperation of gravity measurement device 10. In this manner, acalibration history may be obtained to determine whether there has beenany change in the relative position of tilt meter 14 with respect togravity sensor 12 and, if necessary, recalibration of gravitymeasurement device may be achieved. A method to that end, would includesteps 200, 202, 204, 206, which are identical to steps 100, 102, 104 and106, mentioned above with respect to FIG. 4.

[0040] Referring again to FIG. 5, at step 208, additional tilt data andgravity data are produced. The additional tilt and gravity data may ormay not occur at the same inclination angles X_(m) and Y_(m) from whichthe initial coefficient values are based. At step 210, the additionaltilt and gravity data produced at step 208 are fitted to an additionalpolynomial equation that has a plurality of additional coefficients, a′,b′, c′, d′ and e′, associated therewith. At step 212, an additionalgoodness fit measurement is made of the additional polynomial equationbased upon the additional coefficients a′, b′, c′, d′ and e′. Thegoodness fit measurement may be made employing either the non-normalizedor the normalized chi-square statistic mentioned above. At step 214, theinitial goodness fit measurement is compared with the additionalgoodness fit measurement to determine whether the initial goodness fitmeasurement has a value closer to 1 than the additional goodness fitmeasurement. If this is the case, an initial coefficient is used at step216 and the calibration ends at step 218. Otherwise, the correctionparameters k,ε are derived as a function of the additional coefficientsat step 220. This process may be repeated periodically based upon thepassage of a predetermined amount of time or the occurrence of apredetermined quantity of gravity measurements, or both.

[0041] For the purposes of this specification it will be clearlyunderstood that the word “comprising” means “including but not limitedto”, and that the word “comprises” has a corresponding meaning.

[0042] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for calibrating a subsurface gravitymeasurement device having a tilt meter and a gravity sensor, said methodcomprising: associating tilt information produced by said gravity sensoras a function of tilt information produced by said tilt meter and aninitial correction parameter; producing tilt data with said tilt meter,and gravity data, corresponding to said tilt data, with said gravitydata being produced by said gravity sensor; fitting said tilt data andsaid gravity data to a polynomial equation, with said polynomialequation having a plurality of initial coefficients associatedtherewith, said initial coefficients including information concerningsaid initial correction parameter; and deriving a correction parameteras a function of said initial coefficients.
 2. The method of claim 1wherein deriving said correction parameter further includes determiningvalues for said plurality of initial coefficients using a least-squaresregression.
 3. The method of claim 1 wherein producing tilt data furtherincludes orientating said tilt meter in a plurality of differing angularpositions with respect to a plumb line, defining tilt data, andmeasuring, with said gravity sensor, gravity information at each of saidangular positions, defining said gravity data.
 4. The method of claim 1wherein producing tilt data further includes orientating said tilt meterin at least five differing sets of angular positions with respect tosaid plumb line, defining tilt data, and measuring, with said gravitysensor, five gravity measurements, defining said gravity data.
 5. Themethod of claim 1 wherein deriving said correction parameter furtherincludes determining values for said plurality of initial coefficientsusing a least-squares regression having a weighting function, σ²,applied thereto, and deriving said correction parameter as a function ofsaid initial coefficients.
 6. The method of claim 1 further includingdetermining whether said initial coefficients satisfy a goodness fitcriteria defined by a chi-square statistic, χ².
 7. The method of claim 6further including producing additional tilt data and additional gravitydata and fitting said additional tilt data and said additional gravitydata to an additional polynomial equation having additional coefficientsassociated therewith, upon determining said initial coefficients failedto satisfy said goodness fit criteria, and deriving said correctionparameter as a function of said additional coefficients.
 8. The methodas recited in claim 1 further including determining whether said initialcoefficients satisfy a goodness fit criteria defined by a normalizedchi-square statistic, χ_(v) ², and producing additional tilt data andadditional gravity data and fitting said additional tilt data and saidadditional gravity data to an additional polynomial equation havingadditional coefficients associated therewith, upon determining saidinitial coefficients failed to satisfy said goodness fit criteria, andderiving said correction parameter as a function of said additionalcoefficients.
 9. The method of claim 1 further including determiningwhether said initial coefficients satisfy a goodness fit criteriadefined by a normalized chi-square statistic, χ²/(N−n), where Ncorresponds to a number of data points in said gravity data and ncorresponds to 5, and producing additional tilt data and additionalgravity data and fitting said additional tilt data and said additionalgravity data to an additional polynomial equation having additionalcoefficients associated therewith, upon determining said initialcoefficients failed to satisfy said goodness fit criteria, and derivingsaid correction parameter as a function of said additional coefficients.10. The method of claim 1 wherein said correction parameter includesinformation concerning the difference in an angular deviation said tiltmeter is from said plumb line compared to an angular deviation of saidgravity sensor from said plumb line and further including determining aninitial fit measurement of said initial coefficients and producingadditional tilt data and additional gravity data and fitting saidadditional tilt data and said additional gravity data to an additionalpolynomial equation having additional coefficients associated therewith,and determining an additional goodness fit measurement of saidadditional coefficients and comparing said initial goodness fitmeasurement with said additional goodness fit measurement to determinewhether said angular deviation has changed.
 11. A method forcalibrating, with respect to a plumb line, a gravity measurement devicehaving a tilt meter and a gravity sensor, said method comprising:associating tilt information produced by said gravity sensor as afunction of a relationship between tilt information produced by saidtilt meter and an initial correction parameter; orientating said tiltmeter in a plurality of differing angular positions with respect to saidplumb line, defining tilt data; measuring, with said gravity sensor,gravity information at each of said angular positions, defining gravitydata; fitting said tilt data and said gravity data to a polynomialequation, with said polynomial equation having a plurality of initialcoefficients associated therewith, said coefficients includinginformation concerning said correction parameter; determining values forsaid plurality of initial coefficients' using a least-squaresregression; and deriving a correction parameter as a function of saidcoefficient values.
 12. The method of claim 11 wherein producing tiltdata further includes orientating said tilt meter in at least fivediffering sets of angular positions with respect to said plumb line,defining tilt data, and measuring, with said gravity sensor, fivegravity measurements, defining said gravity data.
 13. The method ofclaim 11 wherein deriving said correction parameter further includesdetermining values for said plurality of initial coefficients using aleast-squares regression having a weighting function, σ², appliedthereto.
 14. The method of claim 11 further including determiningwhether said initial coefficients satisfy a goodness fit criteriadefined by a chi-square statistic, χ².
 15. The method of claim 14further including producing additional tilt data and additional gravitydata and fitting said additional tilt data and said additional gravitydata to an additional polynomial equation having additional coefficientsassociated therewith, upon determining said initial coefficients failedto satisfy said goodness fit criteria, and deriving said correctionparameter as a function of said additional coefficients.
 16. The methodof claim 11 further including determining whether said initialcoefficients satisfy a goodness fit criteria defined by a normalizedchi-square statistic, χ²/(N−n), where N corresponds to a number of datapoints in said gravity data and n corresponds to 5, and producingadditional tilt data and additional gravity data and fitting saidadditional tilt data and said additional gravity data to an additionalpolynomial equation having additional coefficients associated therewith,upon determining said initial coefficients failed to satisfy saidgoodness fit criteria, and deriving said correction parameter as afunction of said additional coefficients.
 17. The method of claim 11wherein said correction parameter includes information concerning thedifference in an angular deviation said tilt meter is from said plumbline compared to an angular deviation of said gravity sensor from saidplumb line and further including determining an initial fit measurementof said initial coefficients and producing additional tilt data andadditional gravity data and fitting said additional tilt data and saidadditional gravity data to an additional polynomial equation havingadditional coefficients associated therewith, and determining anadditional goodness fit measurement of said additional coefficients andcomparing said initial goodness fit measurement with said additionalgoodness fit measurement to determine whether said angular deviation haschanged.
 18. A subsurface gravity measurement device, comprising: abody; a tilt meter connected to said body to produce tilt dataconcerning angular positions said tilt meter forms with respect to adirection of gravity, with said direction of gravity defining a plumbline; a gravity sensor connected to measure said gravity and to produceinformation corresponding thereto, defining gravity data, with saidinformation being a function of an angle said gravity measurement deviceforms with respect to said plumb line, defining tilt information; aprocessor in data communication with both said gravity sensor and saidtilt meter; and a memory in data communication with said processor, saidmemory including a computer readable program to be operated on by saidprocessor that includes a first subroutine to define a relationshipbetween said tilt information produced by said gravity sensor and bothsaid tilt data and an initial correction parameter, and a secondsubroutine to fit said tilt data and said gravity data to a polynomialequation, with said polynomial equation having a plurality of initialcoefficients associated therewith, said initial coefficients includinginformation concerning said correction parameter, and a third subroutineto derive a correction parameter as a function of said initialcoefficients.
 19. The device of claim 18 wherein said subroutine toascertain said correction parameter further includes a code to determinevalues for said plurality of initial coefficients using a least-squaresregression; and derive said correction parameter as a function of saidinitial coefficients.
 20. The device of claim 18 wherein said subroutineto produce tilt data further includes code to orientate said tilt meterat a plurality of differing angular positions with respect to said plumbline, defining tilt data, and code to measure, with said gravity sensor,gravity information at each of said angular positions, defining saidgravity data.
 21. The device of claim 18 further including a subroutineto determine whether said initial coefficients satisfy a goodness fitcriteria defined by a chi-square statistic, χ².
 22. The device of claim21 further including a subroutine to produce additional tilt data andadditional gravity data and fit said additional tilt data and saidadditional gravity data to an additional polynomial equation havingadditional coefficients associated therewith, upon determining saidinitial coefficients failed to satisfy said goodness fit criteria, andderive said correction parameter as a function of said additionalcoefficients.
 23. The device of claim 18 wherein said correctionparameter includes information concerning the difference in an angulardeviation said tilt meter is from said plumb line compared to an angulardeviation of said gravity sensor is from said plumb line and furtherincluding a subroutine to determine an initial fit measurement of saidinitial coefficients and produce additional tilt data and additionalgravity data and fit said additional tilt data and said additionalgravity data to an additional polynomial equation having additionalcoefficients associated therewith, and determine an additional goodnessfit measurement of said additional coefficients and a subroutine tocompare said initial goodness fit measurement with said additionalgoodness fit measurement to determine whether said angular deviation haschanged.